linux-kernel - Re: [PATCH v2] mm: memcontrol: remove obsolete comment of mem_cgroup_unmark_under

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Message-ID: <20200930084343.GO2277@dhcp22.suse.cz>
Date:   Wed, 30 Sep 2020 10:43:43 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     "hannes@...xchg.org" <hannes@...xchg.org>,
        "vdavydov.dev@...il.com" <vdavydov.dev@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: memcontrol: remove obsolete comment of
 mem_cgroup_unmark_under_oom()

On Wed 30-09-20 01:34:25, linmiaohe wrote:
> Michal Hocko <mhocko@...e.com> wrote:
> > On Thu 17-09-20 06:59:00, Miaohe Lin wrote:
> >> Since commit 79dfdaccd1d5 ("memcg: make oom_lock 0 and 1 based rather 
> >> than counter"), the mem_cgroup_unmark_under_oom() is added and the 
> >> comment of the mem_cgroup_oom_unlock() is moved here. But this comment 
> >> make no sense here because mem_cgroup_oom_lock() does not operate on under_oom field.
> >
> >OK, so I've looked into this more deeply and I finally remember why we have this comment here. The point is that under_oom shouldn't underflow and that we have to explicitly check for > 0 because a new child memcg could have been added between mem_cgroup_mark_under_oom and mem_cgroup_unmark_under_oom.
> >
> >So the comment makes sense although it is not as helpful as it could be.
> >I think that changing it to the following will be more usefule
> >
> >	/*
> >	 * Be careful about under_oom underflows becase a child memcg
> >	 * could have neem added after mem_cgroup_mark_under_oom
> 
> Should it be s/neem/been/ ?

yep, fat fingers...

> 
> >	 */
> 
> Many thanks for detailed explanation. Will fix it in v2. Thanks again.

-- 
Michal Hocko
SUSE Labs